1. Technical Field
This invention relates to wireless communications, and has particular relation to how a second finger in a Code Division Multiple Access (CDMA) wireless receiver is to be assigned and deassigned.
2. Background Art
Code Division Multiple Access (CDMA) systems are so named because they use Code, as distinct from frequency or time, to Divide a frequency band into Multiple channels, each of which provides Access to a separate transmitter. The codes cycle through a known sequence. A different offset in the cycle is assigned to each transmitter. When a receiver sets its offset to the same value, it can demodulate the signal from the transmitter. When the values differ to any great extent, the received signal is perceived as only noise. Offset is measured in xe2x80x9cchipsxe2x80x9d. Typical CDMA systems operate at 1.2288 megachips per second. There are 32,767(215xe2x88x921) offsets per cycle, so each cycle takes only 26.6 milliseconds.
A typical receiving station includes several receivers, each of which is known as a xe2x80x9cfingerxe2x80x9d. Typically, a receiving station will have four fingers available. This allows the receiving station to receive signals from up to four different transmitting stations. More commonly, however, a single signal takes several paths (xe2x80x9cmulti-pathxe2x80x9d) from the transmitting station to the receiving station. One path will perhaps be line-of-sight, while a second reflects off a multi-story garage building, while a third reflects off both the building and a vehicle. Because these paths have different lengths, the signals arriving on these paths will have different offsets, perhaps by as much as 20 chips. The receiving station therefore assigns a different finger to each offset, demodulates the signal at each finger, and combines the demodulated signals into a combined signal, ready for further processing.
The typical receiving station also includes a fifth finger, called a xe2x80x9csearcherxe2x80x9d. It is not a true finger, since it is incapable of demodulating a signal. What it is capable of, however, is very quickly measuring whatever signal strength is present at a large number of different offsets. It thus scans for signals at new offsets. It also takes each of the four offsets which already have fingers assigned to them, and measures the signal strength a few chips ahead of, and a few chips behind, the assigned offset.
Suppose that the offset of the received signal has changed. Perhaps the vehicle has moved or the receiving station has moved, or some other change has taken place. This fact can be detected by the signal strength at the leading offset having increased and the signal strength at the lagging offset having decreased (or vice versa). A Time Tracking Loop (TTL) adjusts the finger""s assigned offset accordingly.
There will be some times when signals from different multi-paths converge (for example, the vehicle drives into the garage building), and now only one finger needs to be assigned, with the other being deassigned. There will be other times when a single signal will diverge into different multi-paths (for example, the vehicle drives out of the garage building), and now two fingers need to be assigned.
Conventional methods have not been good in determining when to make an assignment and when to make a deassignment. When making assignment decisions, the searcher determines that there is an energy peak at an offset which is near an offset to which a finger is currently assigned. The searcher measures the gap between the offset of the energy peak and the assigned offset of the finger. If the gap exceeds a threshold, an additional finger is assigned.
Similarly, the gap between the offsets of two different fingers is measured. When the gap falls below a threshold (generally different from the threshold described in the previous paragraph, so as to create hysteresis), the weaker of the two fingers is deassigned.
An apparatus and method are disclosed in which assignment and deassignment of a second finger are made in part by determining the gap, and in part by selectively slowing the Time Tracking Loop (TTL) when appropriate thresholds are crossed.
In its broadest assignment aspect, a method for assigning a second finger in a Code Division Multiple Access (CDMA) wireless receiver includes the steps of:
determining a gap between the assigned offset of a first finger and the offset of an energy peak detected by a searcher;
assigning a second finger if the gap exceeds a first threshold;
slowing down the speed of a Time Tracking Loop for both the first finger and the second finger if the gap exceeds the first threshold but does not exceed a second threshold, which is greater than the first threshold; and
maintaining the speed of the Time Tracking Loop for both the first finger and the second finger if the gap exceeds the second threshold.
In its broadest deassignment aspect, a method for deassigning a second finger in a Code Division Multiple Access (CDMA) wireless receiver includes the steps of:
determining a gap between the assigned offset of a first finger and the assigned offset of the second finger;
deassigning a second finger and maintaining the speed of the Time Tracking Loop for the first finger if the gap is less than a third threshold;
slowing down the speed of a Time Tracking Loop for both the first finger and the second finger if the gap is not less than the third threshold, but is less than a fourth threshold, which is greater than the third threshold; and
maintaining, the speed of the Time Tracking Loop for both the first finger and the second finger if the gap exceeds the fourth threshold.